Channel sounding method in wireless local area network system and apparatus for supporting the same

ABSTRACT

A method for a channel sounding in a wireless local area network. A station receives a 20 MHz null data packet announcement (NDPA) frame and at least one duplicate 20 MHz NDPA frame. The at least one duplicate 20 MHz NDPA frame is a duplicate of the 20 MHz NDPA frame. The station also receives a null data packet (NDP) following the 20 MHz NDPA frame and the at least one duplicate 20 MHz NDPA frame. The bandwidth over which the 20 MHz NDPA frame and the at least one duplicate 20 MHz NDPA frame are transmitted is the same as a bandwidth over which the NDP is transmitted. The station transmits a report frame for the channel sounding. The 20 MHz NDPA frame and the at least one duplicate 20 MHz NDPA frame comprise bandwidth information indicating the bandwidth over which the frames are transmitted.

CROSS-REFERENCE TO RELATED APPLICATIONS

This Application is a Continuation of U.S. application Ser. No.13/880,329 filed Apr. 18, 2013, (now U.S. Pat. No. 9,107,242, issued onAug. 11,2015), which is the National Phase of PCT/KR2011/010133 filed onDec. 27, 2011, which claims priority under 35 U.S.C. §119(e) to U.S.Provisional Application Nos. 61/429,197 filed on Jan. 3, 2011 and61/508,021 filed on Jul. 14, 2011. The contents of all of theseapplications are hereby incorporated by reference as fully set forthherein, in their entirety.

TECHNICAL FIELD

The present invention relates to a Wireless Local Area Network (WLAN)system and, more particularly, to a channel sounding method betweenstations (STAs) in a WLAN system and an apparatus for supporting thesame.

BACKGROUND OF THE INVENTION

With the advancement of information communication technologies, variouswireless communication technologies have recently been developed. Amongthe wireless communication technologies, a wireless local area network(WLAN) is a technology whereby Internet access is possible in a wirelessfashion in homes or businesses or in a region providing a specificservice by using a portable terminal such as a personal digitalassistant (PDA), a laptop computer, a portable multimedia player (PMP),etc.

The IEEE 802.11n is a technical standard relatively recently introducedto overcome a limited data rate which has been considered as a drawbackin the WLAN. The IEEE 802.11n is devised to increase network speed andreliability and to extend an operational distance of a wireless network.More specifically, the IEEE 802.11n supports a high throughput (HT),i.e., a data processing rate of up to above 540 Mbps, and is based on amultiple input and multiple output (MIMO) technique which uses multipleantennas in both a transmitter and a receiver to minimize a transmissionerror and to optimize a data rate.

With the widespread use of the WLAN and the diversification ofapplications using the WLAN, there is a recent demand for a new WLANsystem to support a higher throughput than a data processing ratesupported by the IEEE 802.11n. A next-generation WLAN system supportinga very high throughput (VHT) is a next version of the IEEE 802.11n WLANsystem, and is one of IEEE 802.11 WLAN systems which have recently beenproposed to support a data processing rate of above 1 Gbps in a MACservice access point (SAP).

The next-generation WLAN system supports the transmission of aMulti-User Multiple Input Multiple Output (MU-MIMO) scheme in which aplurality of non-AP STAs accesses a radio channel at the same time inorder to efficiently use the radio channel. According to the MU-MIMOtransmission scheme, an AP can transmit a frame to one or moreMIMO-paired STAs at the same time.

The AP and the plurality of MU-MIMO paired STAs may have differentcapabilities. In this case, a supportable bandwidth, modulation codingscheme (MCS), forward error correction (FEC), etc., may vary dependingon an STA type, usage, channel environment, etc.

In a WLAN system, an Access Point (AP) or an STA or both may obtaininformation about a channel to be used in transmitting a frame to atarget reception AP or STA or both. This may be performed through achannel sounding procedure. That is, a process in which a transmitterrequests channel information to be used for frame transmission andreception from a receiver and the receiver estimates the channel andfeeds the channel information about the channel to the transmitter maybe performed before the transmission and reception of a data frame.Meanwhile, the next-generation WLAN system may receive a greater amountof channel information from a target transmission AP or STA or bothbecause a wider channel bandwidth and an MU-MIMO transmission scheme areadopted. In order to transmit more feedback information, the targettransmission AP or STA or both have to access channels for a longertime.

An AP or an STA or both may do not normally receive necessary controlinformation and data while performing a procedure for channel sounding.In this case, an STA or STAs or both that are intended to estimatechannels may do not estimate the channels or do not transmit a feedbackframe because they do not know whether channel sounding has beenstarted. In this case, an AP or an STA or both that have started channelsounding starts channel sounding again from the beginning because theyhave not received the feedback frame. Accordingly, there are problems inthat an STA or STAs or both that have already estimated channels consumepower due to unnecessary operations and the channels are unnecessarilyoccupied. For this reason, there is a need to introduce a channelsounding method capable of solving the problems.

SUMMARY OF INVENTION

It is an object of the present invention to provide a channel soundingmethod performed by STAs in the next-generation WLAN system supportingMulti-User (MU)-Multiple Input Multiple Output (MIMO).

In an aspect, a method for a channel sounding in a wireless local areanetwork is provided. The method includes transmitting a null data packetannouncement (NDPA) frame, to a plurality of receivers, the NDPA framerequesting a channel statement information feedback and announcing thata null data packet (NDP) frame is to be transmitted; transmitting theNDP frame being a basis of channel estimation for the plurality ofreceivers; receiving a first feedback frame from a first receiver amongthe plurality of receivers, transmitting a feedback poll frame to asecond receiver; and, receiving a second feedback frame from the secondreceiver, the second feedback frame including second channel stateinformation estimated by the second receiver. If the first receiverfails to perform the channel estimation, the first feedback frame is anull feedback frame. The null feedback frame is a feedback frame notincluding channel state information.

The first feedback frame may be transmitted through a bandwidth throughwhich the NDPA frame is transmitted.

The NDP frame may include a length field indicating a time durationwhere the NDP frame is transmitted.

The first feedback frame may be transmitted after the time durationelapses from a point of time at which the NDP frame was transmitted.

The first feedback frame may include a first channel state information.If the first receiver receives the NDPA frame and the NDP frame througha bandwidth narrower than a reference bandwidth through which the NDPAis transmitted, the first channel state information may be channelinformation estimated for the bandwidth.

The first feedback frame may be transmitted through the bandwidth.

The feedback poll frame may include a channel estimation bandwidthindication field indicating a recommended bandwidth for which the secondchannel sate information is estimated.

The feedback poll frame may further include a feedback frame bandwidthindication field indicating a maximum bandwidth through which the secondfeedback frame is transmitted.

The feedback poll frame may further include a reason field indicating atleast one reason why the bandwidth is narrower than the referencebandwidth.

The at least one reason may respectively correspond to each of at leastone reason code.

In another aspect, a wireless apparatus is provided. The apparatusincludes a transceiver transmitting and receiving frames; and aprocessor operationally coupled to the transceiver. The processor isconfigured for: transmitting a null data packet announcement (NDPA)frame, to a plurality of receivers, the NDPA frame requesting a channelstatement information feedback and announcing that a null data packet(NDP) frame is to be transmitted; transmitting the NDP frame being abasis of channel estimation for the plurality of receivers; receiving afirst feedback frame from a first receiver among the plurality ofreceivers; transmitting a feedback poll frame to a second receiver; and,receiving a second feedback frame from the second receiver, the secondfeedback frame including second channel state information estimated bythe second receiver. If the first receiver fails to perform the channelestimation, the first feedback frame is a null feedback frame. The nullfeedback frame is a feedback frame not including channel stateinformation.

According to an embodiment of the present invention, in a channelsounding procedure, if a receiving STA having the highest priority totransmit a feedback normally receives a Null Data Packet Announcement(NDPA) frame transmitted in a duplicate type regarding a specificbandwidth although it has failed in channel estimation, the receivingSTA transmits a null feedback frame to a transmitting STA, such as anAP. When the null feedback frame is received, the AP can perform theremaining channel sounding procedure for other receiving STAs.Accordingly, channel sounding efficiency can be improved as comparedwith the existing method because although the receiving STA having thehighest priority to transmit a feedback fails in channel estimation, theremaining receiving STAs can feed estimated channel state informationback.

According to an embodiment of the present invention, if interferenceoccurs in a specific bandwidth when a receiving STA receives an NDPAframe or an NDP frame or both, a bandwidth for channel state informationand a bandwidth for a feedback frame are controlled. Accordingly,efficiency can be improved because channel sounding can be performed byavoiding interference with a specific bandwidth or channel occupancy byother STAs.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing the configuration of a WLAN system to whichembodiments of the present invention may be applied.

FIG. 2 is a diagram showing the PHY architecture of a WLAN system whichis supported by IEEE 802.11.

FIG. 3 is a diagram showing an example of a PPDU format used in a WLANsystem.

FIG. 4 is a diagram showing a channel sounding method using an NDP inthe next-generation WLAN system.

FIG. 5 is a diagram showing a channel sounding method according to anembodiment of the present invention.

FIG. 6 is a diagram showing another example in which a channel to whichan embodiment of the present invention may be applied is used.

FIG. 7 is a diagram showing yet another example in which a channel towhich an embodiment of the present invention may be applied is used.

FIG. 8 is a diagram showing an example of channel sounding according toan embodiment of the present invention.

FIG. 9 is a block diagram showing a wireless apparatus to which theembodiments of the present invention may be applied.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a diagram showing the configuration of a WLAN system to whichembodiments of the present invention may be applied.

A WLAN system includes one or more Basic Service Set (BSSs). The BSS isa set of stations (STAs) which can communicate with each other throughsuccessful synchronization. The BSS is not a concept indicating aspecific area

An infrastructure BSS includes one or more non-AP STAs STA1, STA2, STA3,STA4, and STA5, an AP (Access Point) providing distribution service, anda Distribution System (DS) connecting a plurality of APs. In theinfrastructure BSS, an AP manages the non-AP STAs of the BSS.

On the other hand, an Independent BSS (IBSS) is operated in an Ad-Hocmode. The IBSS does not have a centralized management entity forperforming a management function because it does not include an AP. Thatis, in the IBSS, non-AP STAs are managed in a distributed manner. In theIBSS, all STAs may be composed of mobile STAs. All the STAs form aself-contained network because they are not allowed to access the DS.

An STA is a certain functional medium, including Medium Access Control(MAC) and wireless-medium physical layer interface satisfying theInstitute of Electrical and Electronics Engineers (IEEE) 802.11standard. Hereinafter, the STA refers to both an AP and a non-AP STA.

A non-AP STA is an STA which is not an AR The non-AP STA may also bereferred to as a mobile terminal, a wireless device, a wirelesstransmit/receive unit (WTRU), a user equipment (UE), a mobile station(MS), a mobile subscriber unit, or simply a user. For convenience ofexplanation, the non-AP STA will be hereinafter referred to the STA.

The AP is a functional entity for providing connection to the DS througha wireless medium for an STA associated with the AP. Althoughcommunication between STAs in an infrastructure BSS including the AP isperformed via the AP in principle, the STAs can perform directcommunication when a direct link is set up. The AP may also be referredto as a central controller, a base station (BS), a node-B, a basetransceiver system (BTS), a site controller, etc.

A plurality of infrastructure BSSs including the BSS shown in FIG. 1 canbe interconnected by the use of the DS. An extended service set (ESS) isa plurality of BSSs connected by the use of the DS. APs and/or STAsincluded in the ESS can communicate with each another. In the same ESS,an STA can move from one BSS to another BSS while performing seamlesscommunication.

In a WLAN system based on IEEE 802.11, a basic access mechanism of amedium access control (MAC) is a carrier sense multiple access withcollision avoidance (CSMA/CA) mechanism. The CSMA/CA mechanism is alsoreferred to as a distributed coordinate function (DCF) of the IEEE802.11 MAC, and basically employs a “listen before talk” accessmechanism. In this type of access mechanism, an AP and/or an STA sensesa wireless channel or medium before starting transmission. As a resultof sensing, if it is determined that the medium is in an idle status,frame transmission starts by using the medium. Otherwise, if it issensed that the medium is in an occupied status, the AP and/or the STAdoes not start its transmission but sets and waits for a delay durationfor medium access.

The CSMA/CA mechanism also includes virtual carrier sensing in additionto physical carrier sensing in which the AP and/or the STA directlysenses the medium. The virtual carrier sensing is designed to compensatefor a problem that can occur in medium access such as a hidden nodeproblem. For the virtual carrier sending, the MAC of the WLAN systemuses a network allocation vector (NAV). The NAV is a value transmittedby an AP and/or an STA, currently using the medium or having a right touse the medium, to anther AP or another STA to indicate a remaining timebefore the medium returns to an available state. Therefore, a value setto the NAV corresponds to a period reserved for the use of the medium byan AP and/or an STA transmitting a corresponding frame.

The IEEE 802.11 MAC protocol, together with a Distributed CoordinationFunction (DCF), provides a Hybrid Coordination Function (HCF) based on aPoint Coordination Function (PCF) of performing periodical polling byusing the DCF and a polling-based synchronous access method so that allreception APs or STAs or both can receive data packets. The HCF includescontention-based Enhanced Distributed Channel Access (EDCA) and HCFControlled Channel Access (HCCA) using a contention-free-based channelaccess scheme employing polling mechanism as access schemes used by aprovider in order to provide data packets to a plurality of users. TheHCF includes a medium access mechanism for improving Quality of Service(QoS) of a WLAN, and QoS data can be transmitted in both a ContentionPeriod (CP) and a Contention-Free Period (CFP).

An AP or an STA or both may perform a procedure of exchanging a RequestTo Send (RTS) frame and Clear To Send (CTS) frame in order to informaccess to a medium. Each of the RTS frame and the CTS frame includesinformation, indicating a scheduled time duration for accessing a radiomedium which is necessary to transmit and receive substantial data frameand, if transmission and reception ACK is supported, an acknowledgement(ACK) frame. Another STA that has received an RTS frame from an AP or anSTA or both trying to transmit frames or has received a CTS frame from atarget STA to which a frame will be transmitted may be set not to accessa medium during a time duration indicated by information included in theRTS/CTS frames. This may be implemented by setting a Network AllocationVector (NAV) during the time duration.

FIG. 2 is a diagram showing the PHY architecture of a WLAN system whichis supported by IEEE 802.11.

The PHY architecture of IEEE 802.11 includes a PHY Layer ManagementEntity (PLME), a Physical Layer Convergence Procedure (PLCP) sublayer210, and a Physical Medium Dependent (PMD) sublayer 200. The PLMEprovides the management function of a physical layer in association witha MAC Layer Management Entity (MLME). The PLCP sublayer 210 transfers aMAC Protocol Data Unit (MPDU), received from a MAC sublayer 220, to thePMD sublayer 200 or transfers a frame, received from the PMD sublayer200, to the MAC sublayer 220 according to an instruction of a MAC layerbetween the MAC sublayer 220 and the PMD sublayer 200. The PMD sublayer200, as a PLCP sublayer, enables the transmission and reception of aphysical entity between two STAs through a radio medium. The MPDUtransmitted by the MAC sublayer 220 is referred to as a Physical ServiceData Unit (PSDU) in the PLCP sublayer 210. The MPDU is similar to thePSDU, but if an Aggregated MPDU (A-MPDU) in which a plurality of MPDUsis aggregated is transferred, each MPDU and each PSDU may be differentfrom each other.

In a process of transferring the PSDU, received from the MAC sublayer220, to the PMD sublayer 200, the PLCP sublayer 210 adds a supplementarysubfield, including information necessary for a physical transceiver, tothe PSDU. The field added to the PSDU may include tail bits necessary torestore a PLCP preamble, a PLCP header, and a convolution encoder to azero state. The PLCP sublayer 210 receives a TXVECTOR parameter,including control information necessary to generate and transmit aPhysical Layer Convergence Procedure (PLCP) Protocol Data Unit (PPDU)and control information necessary for a receiving STA to receive andinterpret the PPDU, from the MAC sublayer 220. The PLCP sublayer 210uses the information included in the TXVECTOR parameter in order togenerate the PPDU including the PSDU.

The PLCP preamble functions to enable a receiver to be prepared for asynchronization function and an antenna diversity before the PSDU istransmitted. A data field may include padding bits, a service fieldincluding a bit sequence for resetting a scrambler, and a coded sequencein which the bit sequence having tail bits added thereto has beenencoded in the PSDU. Here, an encoding scheme may be either a BinaryConvolutional Coding (BCC) encoding scheme or a Low Density Parity Check(LDPC) encoding scheme according to an encoding scheme supported by anSTA that receives a PPDU. The PLCP header includes a field includinginformation about a PLCP Protocol Data Unit (PPDU) to be transmitted.The PLCP header will be described in more detail later with reference toFIG. 3.

The PLCP sublayer 210 generates the PPDU by adding the field to the PSDUand transmits the generated PPDU to a receiving STA via the PMD sublayer200. The receiving STA receives the PPDU, obtains information necessaryto restore data from a PLCP preamble and a PLCP header, and restores thedata. The PLCP sublayer of the receiving STA transfers an RXVECTORparameter, including control information included in a PLCP preamble anda PLCP header, to an MAC sublayer so that the MAC sublayer can interpretthe PPDU and obtain data in a reception state.

Unlike the existing WLAN system, the next-generation WLAN systemrequires a higher throughput which is called a Very High Throughput(VHT). To this end, the next-generation WLAN system tries to support 80MHz bandwidth transmission, contiguous 160 MHz bandwidth transmission,non-contiguous 160 MHz bandwidth transmission or higher. Furthermore, anMU-MIMO transmission method is provided for a higher throughput. The APof the next-generation WLAN system can transmit a data frame to one ormore MIMO-paired STAs at the same time.

In a WLAN system, such as that shown in FIG. 1, an AP 10 can transmitdata to an STA group, including at least one of a plurality of STAs 21,22, 23, 24, and 30 associated therewith, at the same time. In a WLANsystem, such as that shown in FIG. 1, the AP 10 may transmit data to anSTA group including at least one STA, from among the plurality of STAs21, 22, 23, 24, and 30 associated therewith, at the same time. Anexample where the AP performs MU-MIMO transmission to the STAs is shownin FIG. 1. In a WLAN system supporting Tunneled Direct Link Setup(TDLS), Direct Link Setup (DLS), or a mesh network, however, an STAtrying to send data may send a PPDU to a plurality of STAs by using theMU-MIMO transmission scheme. An example where an AP sends a PPDU to aplurality of STAs according to the MU-MIMO transmission scheme isdescribed below.

The data respectively transmitted to each of the STAs may be transmittedthrough different spatial streams. The data packet transmitted by the AP10 may be a PPDU, generated and transmitted by the physical layer of aWLAN system, or a data field included in the PPDU, and the data packetmay be referred to as a frame. In an example of the present invention,it is assumed that a target transmission STA group MU-MIMO-paired withthe AP 10 includes the STA 1 21, the STA 2 22, the STA 3 23, and the STA4 24. Here, data may not be transmitted to a specific STA of the targettransmission STA group because spatial streams are not allocated to thespecific STA. Meanwhile, it is assumed that the STAa 30 is associatedwith the AP 10, but not included in the target transmission STA group.

In order to support MU-MIMO transmission in a WLAN system, an identifiermay be allocated to a target transmission STA group, and the identifiermay be called a group ID. An AP transmits a group ID management frame,including group definition information, to STAs supporting MU-MIMOtransmission in order to allocate a group ID to the STAs. The group IDis allocated to the STAs based on the group ID management frame prior toPPDU transmission. A plurality of group IDs may be allocated to one STA.

Table 1 below shows information elements included in the group IDmanagement frame.

TABLE 1 Order Information 1 Category 2 VHT action 3 Membership status 4Spatial stream position

The frames of the category field and the VHT action field correspond tomanagement frames. The category field and the VHT action field are setto identify that the relevant frames are group ID management frames usedin the next-generation WLAN system supporting MU-MIMO.

As in Table 1, group definition information includes the membershipstatus information, indicating whether an STA belongs to a specificgroup ID, and spatial stream position information indicating that whatplace is the spatial stream set of a relevant STA located from all thespatial streams according to MU-MIMO transmission if the STA belongs tothe relevant group ID.

Since a plurality of group IDs is managed by one AP, membership statusinformation provided to one STA needs to indicate whether the STAbelongs to each of the group IDs managed by the AP. Accordingly, themembership status information may exist in an array form of subfields,indicating whether the STA belongs to each group ID. The spatial streamposition information may exist in an array form of subfields, indicatinga position of a spatial stream set occupied by an STA regarding eachgroup ID, because the spatial stream position information indicates aposition for each group ID. Furthermore, the membership statusinformation and the spatial stream position information for one group IDmay be implemented within one subfield.

If an AP transmits a PPDU to a plurality of STAs according to theMU-MIMO transmission scheme, the AP includes information, indicating agroup ID, in the PPDU, and transmits the information as controlinformation. When an STA receives the PPDU, the STA checks whether it isa member STA of a target transmission STA group by checking a group IDfield. If the STA is checked to be a member of the target transmissionSTA group, the STA may check that what place is a spatial stream set,transmitted thereto, placed from all the spatial streams. Since the PPDUincludes information about the number of spatial streams allocated to areception STA, the STA can receive data by searching for spatial streamsallocated thereto.

FIG. 3 is a diagram showing an example of a PPDU format used in a WLANsystem.

Referring to FIG. 3, a PPDU 300 may include an L-STF 310, an L-LTF 320,an L-SIG field 330, a VHT-SIG A field 340, a VHT-STF 350, a VHT-LTF 360,a VHT-SIG B field 370, and a data field 380.

The PLCP sublayer forming the physical layer converts a PSDU, receivedfrom the MAC layer, into the data field 380 by adding necessaryinformation to the PSDU, generates the PPDU 300 by adding fields, suchas the L-STF 310, the L-LTF 320, the L-SIG field 330, the VHT-SIG Afield 340, the VHT-STF 350, the VHT-LTF 360, and the VHT-SIGB field 370,to the data field 380, and transmits the PPDU 300 to one or more STAsthrough the PMD sublayer forming the physical layer. Control informationnecessary for the PLCP sublayer to generate the PPDU and controlinformation, included in the PPDU and transmitted so that a receivingSTA can use the control information to interpret the PPDU, are providedfrom the TXVECTOR parameter received from the MAC layer.

The L-STF 310 is used for frame timing acquisition, Automatic GainControl (AGC) convergence, coarse frequency acquisition, etc.

The L-LTF 320 is used for channel estimation for demodulating the L-SIGfield 330 and the VHT-SIG A field 340.

The L-SIG field 330 is used for an L-STA to receive the PPDU 300 andobtain data by interpreting the PPDU 300. The L-SIG field 330 includes arate subfield, a length subfield, parity bits, and a tail field. Therate subfield is set to a value indicating a bit rate for data to betransmitted now.

The length subfield is set to a value indicating the octet length of aPhysical Service Data Unit (PSDU) that the MAC layer requests a physicallayer to send the PSDU. Here, an L_LENGTH parameter related toinformation about the octet length of the PSDU is determined on thebasis of a TXTIME parameter related to transmission time. TXTIMEindicates a transmission time determined by the physical layer in orderto transmit a PPDU including the PSDU, in response to a transmissiontime that the MAC layer has requested the physical layer to send thePSDU. Since the L_LENGTH parameter is a parameter related to time, thelength subfield included in the L-SIG field 330 includes informationrelated to the transmission time.

The VHT-SIG A field 340 includes control information (or signalinformation) necessary for STAs, receiving the PPDU 300, to interpretthe PPDU 300. The VHT-SIG A field 340 is transmitted through two OFDMsymbols. Accordingly, the VHT-SIG A field 340 may be divided into aVHT-SIG Al field and a VHT-SIG A2 field. The VHT-SIG A1 field includeschannel bandwidth information used for PPDU transmission, informationindicating whether Space Time Block Coding (STBC) is used, informationindicating a scheme for transmitting a PPDU, from among SU and MU-MIMOschemes, information indicating a target transmission STA groupincluding a plurality of STAs that are MU-MIMO-paired with an AP if thetransmission scheme is the MU-MIMO scheme, and information about spatialstreams allocated to each STA of the target transmission STA group. TheVHT-SIG A2 field includes short Guard Interval (GI)-related information.

The information indicating the MIMO transmission scheme and theinformation indicating the target transmission STA group may beimplemented into a piece of MIMO indication information. For example,they may be implemented in the form of a group ID. The group ID may beset to a value having a specific range. A specific value of the rangemay indicate the SU-MIMO transmission scheme, and the remaining valuesof the range may be used as an identifier for a relevant targettransmission STA group if the PPDU 300 is transmitted according to theMU-MIMO transmission scheme.

If the group ID indicates that the PPDU 300 is transmitted according tothe SU-MIMO transmission scheme, the VHT-SIG A2 field includes codingindication information, indicating whether a coding scheme applied to adata field is a Binary Convolution Coding (BCC) scheme or a Low DensityParity Check (LDPC) coding scheme, and Modulation Coding Scheme (MCS)information about a channel between a sender and a recipient.Furthermore, the VHT-SIG A2 field may include the AID of an STA to whichthe PPDU 300 will be transmitted or a partial AID including some bitsequences of the AID or both.

If the group ID indicates that the PPDU 300 is transmitted according tothe MU-MIMO transmission scheme, the VHT-SIG A field 340 includes codingindication information indicating whether a coding scheme applied to adata field to be transmitted to reception STAs that are MU-MIMO-pairedis the BCC scheme or the LDPC coding scheme. In this case, ModulationCoding Scheme (MCS) information for each reception STA may be includedin the VHT-SIG B field 370.

The VHT-STF 350 is used to improve AGC estimation performance in MIMOtransmission.

The VHT-LTF 360 is used for an STA to estimate a MIMO channel. TheVHT-LTF 360 may be set to the number corresponding to the number ofspatial streams through which the PPDU 300 is transmitted because thenext-generation WLAN system supports MU-MIMO. Additionally, full channelsounding is supported. If the full channel sounding is performed, thenumber of VHT-LTFs may be further increased.

The VHT-SIG B field 370 includes dedicated control information which isnecessary for a plurality of MIMO-paired STAs to obtain data byreceiving the PPDU 300. Accordingly, only when common controlinformation included in the VHT-SIG B field 370 indicates that thereceived PPDU 300 has been subjected to MU-MIMO transmission, an STA maybe designed to decode the VHT-SIG B field 370. On the other hand, if thecommon control information indicates that the received PPDU 300 is for asingle STA (including SU-MIMO), an STA may be implemented not to decodethe VHT-SIG B field 370.

The VHT-SIG B field 370 includes information about an MCS andinformation about rate matching for each STA. The VHT-SIG B field 370further includes information indicating the length of a PSDU which isincluded in a data field for each STA. The information indicating thelength of the PSDU is information indicating the length of the bitsequence of the PSDU and may be indicated by an octet unit. The size ofthe VHT-SIG B field 370 may vary an MIMO transmission type (MU-MIMO orSU-MIMO) and a channel bandwidth used for PPDU transmission.

The data field 380 includes data intended to be transmitted to an STA.The data field 380 includes a service field for resetting a PLCP ServiceData Unit (PSDU) to which an MAC Protocol Data Unit (MPDU) in the MAClayer has been transferred and a scrambler, a tail field including a bitsequence necessary to restore a convolution encoder to a zero state, andpadding bits for normalizing the length of a data field.

In a WLAN system, such as that shown in FIG. 1, if the AP 10 intends totransmit data to the STA 1 21, the STA 2 22, and the STA 3 23, the AP 10may transmit the PPDU to an STA group including the STA 1 21, the STA 222, the STA 3 23, and the STA 4 24. In this case, the data may betransmitted in such a manner that spatial streams are not allocated tothe STA 4 24 and a specific number of spatial streams are allocated toeach of the STA 1 21, the STA 2 22, and the STA 3 23, as in FIG. 2. Inthe example of FIG. 2, it can be seen that one spatial stream has beenallocated to the STA 1 21, three spatial streams have been allocated tothe STA 2 22, and two spatial streams have been allocated to the STA 323.

One of the most important characteristics of the next-generation WLANsystem is that it supports the MU-MIMO transmission scheme in whichseveral spatial streams are transmitted to a plurality of STAs by usingmultiple antennas. In this case, the overall throughput of the systemcan be improved. In an environment including a plurality of STAs, an APtrying to transmit data transmits a Physical Layer Convergence Procedure(PLCP) Protocol Data Unit (PPDU) through a beamforming procedure so asto transmit the data to a target transmission STA group. Accordingly, anAP or an STA or both trying to transmit PPDUs using the MU-MIMOtransmission scheme are required to perform channel sounding in order toobtain channel information about each target transmission STA.

Channel sounding for MU-MIMO may be initiated by a transmitter thattries to transmit a PPDU by forming a beam. The transmitter may becalled a beamformer, and a receiver may be called a beamformee. In aWLAN system supporting Downlink (DL) MU-MIMO, an AP has a position of atransmitter (i.e., a beamformer) and initiates channel sounding. An STAhas a position of a receiver (i.e., a beamformee), estimates a channelaccording to channel sounding initiated by an AP, and reports channelinformation. In describing a detailed channel sounding methodhereinafter, channel sounding in DL MU-MIMO transmission is assumed. Thefollowing channel sounding method may be applied to a wirelesscommunication system supporting common MU-MIMO transmission.

Channel sounding in the next-generation WLAN system is performed on thebasis of a Null Data Packet (NDP). The NDP has a PPDU format without adata field. An STA performs channel estimation on the basis of the NDPand feeds channel state information (i.e., the result of the estimation)back to an AR The NDP may be called a sounding frame. Channel soundingbased on the NDP is described with reference to FIG. 4.

FIG. 4 is a diagram showing a channel sounding method using an NDP inthe next-generation WLAN system. In this example, an AP 410 performschannel sounding on three target transmission STAs 421, 422, and 423 inorder to transmit data to the three target transmission STAs. In someembodiments, the AP may perform channel sounding on one STA.

Referring to FIG. 4, the AP 410 transmits an NDP Announcement (NDPA)frame to the STA1 421, the STA2 422, and the STA3 423 at step S410. TheNDPA frame informs that channel sounding will be initiated and an NDPwill be transmitted. The NDPA frame may be called a soundingannouncement frame.

The NDPA frame includes information for identifying an STA which willestimate a channel and transmit a feedback frame, including channelstate information, to an AR In other words, the STA determines whetherit is an STA participating in channel sounding by receiving the NDPAframe. Accordingly, the AP 410 includes an STA information field,including information about a target sounding STA, in the NDPA frame andthen transmits the NDPA frame. The STA information field may be includedfor every target sounding STA.

This is for informing information for identifying an STA that willtransmit a feedback frame in response to a next transmitted NDP.

If the NDPA frame is transmitted to one or more target STAs for MU-MIMOchannel sounding, the AP 410 broadcasts the NDPA frame. If the NDPAframe is transmitted to one target STA for SU-MIMO channel sounding, theAP 410 may set receiver address information about the NDPA frame as theMAC address of the target STA and transmit the NDPA frame in a unicastmanner.

Table 2 shows an example of STA information field formats included inthe NDPA frame.

TABLE 2 Subfield Description AID Contains the AID of the STA expected toprocess the following NDP frame and prepare the sounding feedback.Feedback type Indicates the type of feedback requested. Set to 0 for SU.Set to 1 for MU. Nc index Nc Index Indicates the feedback dimensionrequested if the Feedback Type field is 1: Set to 0 to request Nc = 1Set to 1 to request Nc = 2 . . . Set to 7 to request Nc = 8 Reserved ifthe Feedback Type field is 0.

In Table 1, Nc indicates the number of columns of a beamforming feedbackmatrix from feedback information transmitted to an AP in response to anNDP frame by a target sounding STA that has received the NDP frame.

STAs that have received an NDPA frame may check whether they are targetsounding STAs by checking an AID subfield value included in an STAinformation field. In an embodiment, such as that shown in FIG. 4, theNDPA frame may include an STA information field including the AID of theSTAT 421, an STA information field including the AID of the STA2 422,and an STA information field including the AID of the STA3 423.

After transmitting the NDPA frame, the AP 410 transmits an NDP frame tothe target STAs at step S420. The NDP frame may have a format having adata field omitted from a PPDU format, such as that shown in FIG. 3. TheAP 410 precodes the NDP frame on the basis of a specific precodingmatrix and transmits the NDP frame to the target sounding STAs.Accordingly, the target sounding STAs 421, 422, and 423 estimatechannels on the basis of the VHT-LTF of the NDP and obtain channel stateinformation.

As pieces of control information included in the NDP frame when the NDPframe is transmitted, length information, indicating the length of aPSDU included in a data field or the length of an Aggregate-MAC ProtocolData Unit (A-MPDU) included in the PSDU, is set to 0, and informationindicating the number of target transmission STA to which the NDP framewill be transmitted is set to 1. Furthermore, a group ID, indicatingwhether a transmission scheme used to transmit the NDP frame is MU-MIMOor SU-MIMO and a target transmission, is set as a value indicatingSU-MIMO transmission. Information indicating the number of spatialstreams allocated to a target transmission STA is set to indicate thenumber of spatial streams transmitted to the target transmission STAthrough MU-MIMO or SU-MIMO. Information about a channel bandwidth usedto transmit the NDP frame may be set as a bandwidth value used totransmit the NDPA frame.

The STA1 421 transmits a feedback frame to the AP 410 at step S431. Achannel bandwidth used to transmit the feedback frame may be set to benarrower than or equal to the channel bandwidth used to transmit theNDPA frame.

After receiving the feedback frame from the STA1 421, the AP 410transmits a feedback poll frame to the STA2 422 at step S441. Thefeedback poll frame is a frame for requesting a receiving STA totransmit a feedback frame. The feedback poll frame is transmitted to anSTA that will be requested to transmit a feedback frame in a unicastmanner. The STA2 422 that has received the feedback poll frame transmitsa feedback frame to the AP 410 at step S432. Next, the AP 410 transmitsa feedback poll frame to the STA3 423 at step S442. The STA3 423transmits a feedback frame to the AP 410 in response to the feedbackpoll frame at step S433.

In a WLAN system, a channel bandwidth used to transmit data may bevarious. In order to estimate channels for various bandwidths, pieces ofchannel information for the various bandwidths may be fed back. Thenext-generation WLAN system supports 20 MHz, 40 MHz, 80 MHz, contiguous160 MHz, and non-contiguous 160 MHz (80+80) MHz bandwidths. Accordingly,the amount of channel feedback information may be increased becausepieces of channel information about the bandwidths are fed back.

In the present invention, channel state information according to channelestimation performed by an STA is included in a feedback frametransmitted from the STA to an AP and then transmitted. The channelstate information of the feedback frame may be implemented using achannel information field and a channel information control field. Table3 and Table 4 below show the formats of the channel information controlfield and the channel information field.

TABLE 3 Subfield Description Nc index Indicate the number of columns ofa beamforming feedback matrix when Nc = 1, 0 when Nc = 2, 1 . . . whenNc = 8, 7 Nr index Indicate the number of rows of a beamforming feedbackmatrix when Nr = 1, 0 when Nr = 2, 1 . . . when Nr = 8, 7 Channelbandwidth Indicate the bandwidth of an estimated channel when 20 MHz, 0when 40 MHz, 1 when 80 MHz, 2 when 160 MHz or 80 + 80 MHz, 3 Grouping(Ng) Indicate the number of carriers for grouping when Ng = 1, 0 when Ng= 2, 1 when Ng = 4, 2 (3 is set for reservation) Codebook in- Indicatethe size of codebook entries formation MU-scheme Indicate whether it isfor beamforming feedback for SU-MIMO or MU-MIMO Sounding sequenceIndicate a sequence number from an NPDA frame requesting a feedback

TABLE 4 Subfield Description Signal to Noise Ratio An average SNR onsubcarriers in a (SNR) of a spatial stream 1 receiver for the firstspatial stream . . . . . . SNR of a spatial stream Nc An average SNR onsubcarriers in a receiver for the Nc^(th) spatial stream Beamformingfeedback matrix Order of an angle of a beamforming (subcarrier index 0)feedback matrix for a relevant subcarrier Beamforming feedback matrixOrder of an angle of a beamforming (subcarrier index 1) feedback matrixfor a relevant subcarrier . . . . . . Beamforming feedback matrix Orderof an angle of a beamforming (subcarrier index Ns) feedback matrix for arelevant subcarrier

The pieces of information about the channel information fields listed inTable 4 may be interpreted on the basis of the pieces of informationincluded in the channel control field listed in Table 3.

Meanwhile, in a WLAN system supporting MU-MIMO, when channel sounding isinitiated on the basis of an NDP frame, a target sounding STA may do notnormally receive the NDP frame. In this case, the target sounding STAcannot transmit a feedback frame, including channel state information,to an AP because it cannot normally perform channel sounding on thebasis of the NDP frame. In this case, the AP newly initiates channelsounding.

Initiating channel sounding again as described above may cause a problemin channel sounding for MU-MIMO. In channel sounding applied to aplurality of target sounding STAs, if channel sounding is not normallyperformed by one STA, inefficiency is problematic because overallchannel sounding has to be separately performed again. The presentinvention proposes a channel sounding method capable of improvinginefficiency in channel sounding due to problems occurring in thetransmission and reception of an NDP frame.

While an AP performs channel sounding, a wireless channel environmentmay be changed owing to lots of factors, such as signal fading and aninterference signal. Accordingly, an STA has received an NDPA frameincluding information indicating that the STA itself is a targetsounding STA, but may not normally receive an NDP frame transmittedsubsequently to the NDPA frame. Whether the NDP frame has been normallyreceived may be determined by the CRC of an L-SIG field or a VHT-SIGAfield or may be determined by whether a specific agreed bit sequenceused in the VHT-SIGB field of the NDP frame has been normally received.The STA has normally received a radio signal for the NDP frame, but mayfail in the demodulation of the NDP frame. In this case, the STA cannotestimate a channel and obtain channel state information because itcannot obtain the number of spatial streams allocated thereto andinformation related to a bandwidth.

The present invention proposes a method of transmitting an additionalframe in order to inform that the STA does not have channel stateinformation to be fed back to an AP, unlike in the existing method ofnot transmitting any frame. If an AP does not receive a feedback framein expected timing, the AP transmits an NDPA frame in order to start newchannel sounding.

If an STA that does not feed channel state information back to an AP isdesignated as an STA that will transmit a feedback frame to the AP forthe first time in response to an NDPA frame, the AP transmits the NDPAframe in order to initiate new channel sounding. In this case,inefficiency is further increased because pieces of channel stateinformation obtained by other STAs through channel estimation aredisused. Accordingly, as the remaining channel sounding procedure isperformed, the STA may transmit a null feedback frame, not includingchannel state information, to the AP. This is described in detail withreference to FIG. 5.

FIG. 5 is a diagram showing a channel sounding method according to anembodiment of the present invention.

Referring to FIG. 5, an AP 510 transmits an NDPA frame at step S510 andthen transmits an NDP frame at step S520. The steps S510 and S520 arethe same as those of the steps S410 and S420 of FIG. 4, and a detaileddescription thereof is omitted.

It is assumed that an STA1 521 has not normally received the NDP frame.The STA1 521 cannot obtain channel state information through channelestimation. Accordingly, the STA1 521 transmits a null feedback frame,not including channel state information, to the AP 510 at step S530.

The STA1 521 may do not obtain information necessary to transmit thenull feedback frame because it has not normally received the NDP frame.Accordingly, the present invention proposes a method in which an STArefers to an NDPA frame transmitted earlier than an NDP frame in orderto obtain information necessary to transmit a null feedback framealthough the STA has not normally received the NDP frame. The STA1 521may obtain information necessary to transmit the null feedback frame onthe basis of information obtained by receiving the NDPA frame.

An example in which the STA1 521 has not normally received the NDP framemay include a case where the STA1 521 has not decoded a VHT-SIGA fieldincluded in the NDP frame. The STA1 521 cannot know information about achannel bandwidth and information about the number of allocated spatialstreams. Meanwhile, the STA1 521 may analogize the total length of theNDP frame based on length information about the L-SIG field of the NDPframe. Accordingly, the STA1 521 may know that a point of time at whichthe transmission of a feedback frame was finished is from a point oftime at which the AP finished transmitting the NDP frame to a point oftime after a Short InterFrame Space (SIFS). The point of time at whichthe transmission of a feedback frame was finished is different from apoint of time at which an STA having a lower priority, such as an STA2522 and an STA3 523, transmits a feedback frame and at which thetransmission of the feedback frame is determined in response to afeedback poll frame.

A channel bandwidth used to transmit the NDP frame is equal to a channelbandwidth used to transmit the NDPA frame. Accordingly, a bandwidth usedto transmit a null feedback frame may be determined based on theparameter CH_BANDWIDTH of RXVECTOR (i.e., an information parameterobtained by receiving the NDPA frame transmitted earlier than the NDPframe). The STA1 521 may determine a bandwidth for transmitting the nullfeedback frame and set the parameter CH_BANDWIDTH of TXVECTOR (i.e., atransmission information parameter).

As described above, the STA1 521 can transmit the null feedback framebecause it can determine timing when the null feedback frame istransmitted and the parameter CH_BANDWIDTH although the STA1 521 has notnormally received the NDP frame.

The AP 510 does not initiate new channel sounding and performs theremaining channel sounding for the STA2 522 and the STA3 523 because theAP 510 has not received channel state information from the STA1 521, buthas received the null feedback frame therefrom.

After receiving the null feedback frame from the STA1 521, the AP 510transmits a feedback poll frame to the STA2 522 at step S541. Thefeedback poll frame is a frame that requests a receiving STA to transmita feedback frame. The feedback poll frame is transmitted to an STA thatwill be requested to transmit the feedback frame in a unicast manner.The STA2 522 that has received the feedback poll frame transmits afeedback frame to the AP 510 at step S551. Next, the AP 510 transmits afeedback poll frame to the STA3 523 at step S542. In response to thefeedback poll frame, the STA3 523 transmits a feedback frame to the AP510 at step S552.

In addition, if the feedback frame is not received from the STA1 521,the AP 510 may selectively request channel state information from theremaining STAs or perform a procedure of starting a new channel soundingprocedure.

Meanwhile, in a WLAN according to the IEEE 802.11 standard, frames maybe classified into three types; a data frame, a control frame, and amanagement frame. In general, when a control frame is transmitted inresponse to a specific received frame, the same bandwidth as a bandwidthspanned by the received control frame is spanned and transmitted.

In a system using an Industrial, Scientific, and Medical (ISM) band asopportunity permits, interference not known to a transmitter may existon the receiver side. In particular, in a wide frequency band, systemsaccess channels while coexisting in various communication systems andoperate using different bandwidth configurations. In this case, afrequency selective and strong interference signal may be detected by areceiver, but may not be detected by a transmitter.

The present invention proposes a protocol in which in channel sounding,an AP transmits an NDPA frame and an NDP frame and STAs transmitfeedback frames through a bandwidth which is smaller than or equal to abandwidth used to transmit the NDPA frame and the NDP frame.

FIG. 6 is a diagram showing another example in which a channel to whichan embodiment of the present invention may be applied is used. FIG. 6 isan example in which a channel is used from a viewpoint of a beamformer,such as an AP.

Referring to FIG. 6, an NDPA frame includes a primary subchannel, andthe NDPA frame is transmitted through four subchannels each having a 20MHz bandwidth. The NDPA frame is transmitted according to a duplicateframe structure. Accordingly, although an AP transmits the NDPA frame byspanning a large bandwidth, an STA can normally decode and understandthe NDPA frame only if the STA normally receives only some bands.

The AP transmits the NDP frame by spanning a bandwidth based on abandwidth spanned for the transmission of the NDPA frame.

The AP receives a feedback frame through a bandwidth narrower than thebandwidth spanned for the transmission of the NDPA frame and the NDPframe.

FIG. 7 is a diagram showing yet another example in which a channel towhich an embodiment of the present invention may be applied is used.

Referring to FIG. 7, an NDPA frame and an NDP frame are transmittedaccording to a duplicate frame structure through four subchannels whichis a total 80 MHz bandwidth. However, interference at a receiving stage(Rx interference) was generated in a specific 40 MHz bandwidth from thetotal 80 MHz bandwidth through which the NDPA frame and the NDP frameare transmitted.

In this case, an STA can decode the NDPA frame per 20 MHz because theNDPA frame is transmitted according to the duplicate frame structure. Ifthe STA has decoded the NDPA frame for a specific bandwidth, the STA canperform channel estimation on the specific bandwidth and transmit afeedback frame although the NDP frame is received through an originalchannel band intended by an AP. In case of FIG. 7, the STA performschannel estimation for the 40 MHz bandwidth and generates channel stateinformation.

In addition, the present invention proposes a method in which an STAincludes channel estimation information in a feedback frame andtransmits the feedback frame if a primary subchannel is included in abandwidth through which the STA normally receives an NDPA frame or anNDP frame or both in transmitting the feedback frame. Furthermore, whentransmitting the feedback frame, the STA may use a bandwidth throughwhich the NDPA frame or the NDP frame or both have been normallyreceived.

In this channel sounding, although the NDPA frame is transmittedaccording to a duplicate frame structure, the STA may need informationabout a bandwidth used to transmit the NDPA frame. The information aboutthe bandwidth may be included in the service field of a data field. Insome embodiments, the information about the bandwidth may be included inthe frame body of the NDPA frame itself and then transmitted.

Meanwhile, if channel sounding is performed on a plurality of STAs, abandwidth supported by each of the STAs may be different. Furthermore,an AP which has received a feedback frame from the first STA may want tocontrol a bandwidth through which channel state information for anotherbandwidth or a feedback frame or both are transmitted in relation to theremaining STAs. In this case, information related to a maximum bandwidththrough which the feedback frame can be transmitted or maximum bandwidthinformation related to channel state information or both may be includedin an NDPA frame or a feedback poll frame or both. The informationrelated to a maximum bandwidth through which the feedback frame can betransmitted may be implemented by setting the service field of the datafield of a PPDU. The maximum bandwidth information related to channelestimation for channel state information may be implemented by addingthe fields of an NDPA frame or a feedback poll frame or both.

FIG. 8 is a diagram showing an example of channel sounding according toan embodiment of the present invention. It is assumed that an STA1 821is an STA that transmits a feedback frame with the first priority.

Referring to FIG. 8, an AP 810 transmits an NDPA frame to the STA1 821and an STA2 822 at step S810. Next, the AP 810 transmits a NDP frame tothe STA1 821 and the STA2 822 at step S820. The NDPA frame istransmitted according to a duplicate frame structure while spanning a160 MHz bandwidth. The NDP frame is also transmitted while spanning a160 MHz bandwidth.

The NDPA frame includes information that the NDPA frame spans the 160MHz bandwidth. It is assumed that when receiving the NDPA frame, theSTA1 821 has not normally received the NDPA frame for the entire 160 MHzbandwidth, but has normally received the NDPA frame for an 80 MHzbandwidth, including a primary subchannel, because interference hasoccurred in a specific band including a non-primary subchannel.

The STA1 810 performs channel estimation based on the NDP frame for the80 MHz bandwidth and feeds channel state information about the 80 MHzbandwidth back to the AP 810 at step S830. At this time, the feedbackframe including the channel state information may be transmitted throughthe 80 MHz bandwidth.

In order to receive channel state information from the STA2 822, the AP810 transmits a feedback poll frame to the STA2 822 at step S840. Inorder to limit a bandwidth through which the feedback frame of the STA2822 is transmitted, the AP 810 may include information about channelestimation for the 80 MHz bandwidth in the feedback poll frame.

The STA2 822 performs channel estimation on the 80 MHz bandwidthindicated by the information included in the feedback poll frame andtransmits the feedback frame, including channel state information, tothe AP 810 at step S850. A bandwidth used to transmit the feedback framemay be the bandwidth determined to be available for the STA2 822. If theSTA2 822 has normally received the NDPA frame and the NDP frame throughthe 160 MHz bandwidth, the STA2 822 may transmit the feedback frame byusing the 160 MHz bandwidth. If the STA2 822 has normally received theNDPA frame for only the 80 MHz bandwidth like the STA1 821, the STA2 822may transmit the feedback frame by using the 80 MHz bandwidth. Ifmaximum bandwidth information for transmitting the feedback frame isincluded in the received feedback poll frame, the STA2 822 may transmitthe feedback frame by using a bandwidth smaller than or equal to themaximum bandwidth on the basis of maximum bandwidth information.

If STAs feed back channel state information about a bandwidth smallerthan a bandwidth through which an NDPA frame or an NDP frame or both aretransmitted, each STA may include information indicative of a reason ina feedback frame and transmit the feedback frame. The informationindicative of the reason is a reason code and may be implemented byinforming the reason code using previously agreed information. Thereason code may be given as in Table 5 below.

TABLE 5 Reason Code Description . . . . . . 52 Partial bandwidthSounding feedback due to CCA busy (Energy Detection) on non-primarysubchannel 53 Partial bandwidth Sounding feedback due to CCA busy (Valid802.11 signal) on non-primary subchannel 54 Partial bandwidth Soundingfeedback due to miss- detection of non-primary subchannels of the NDPAand the NDP 55 Partial bandwidth Sounding feedback due to un- supportedSounding BW . . . . . .

In addition, if the reason code indicates that a channel occupancy state(i.e., a CCA busy state) has been detected in a subchannel, such as 20,40, 80, 160, or 80+80 MHz, or that an NDPA frame or an NDP frame has notbeen normally received, an STA may include information indicative of arelevant subchannel in a feedback frame and transmit the feedback frame.Furthermore, if a CCA busy state is detected or an NDPA frame or an NDPframe or both is not normally received, detailed information about arelevant reason may be further included in the feedback frame. Forexample, whether an energy detection CCA busy state has continued beforean NDPA frame is received, whether a valid 802.11 signal CCA busy statehas continues before an NDPA frame is received, or whether an NDPA frameor an NDP frame or both have not been received and decoded in a specificsubchannel when they are received may be classified and informed.

FIG. 9 is a block diagram showing a wireless apparatus to which theembodiments of the present invention may be applied. The wirelessapparatus may be an AP or an STA.

The wireless apparatus 900 includes a processor 910, memory 920, and atransceiver 930. The transceiver 930 transmits and receives radiosignals, and the physical layer of IEEE 802.11 is implemented in thetransceiver 930. The processor 910 is functionally coupled to thetransceiver 930, and it implements the MAC layer and the physical layerof IEEE 802.11. The processor 910 may be set to generate and transmitthe NDPA frame, the NDP frame, and the feedback poll frame proposed bythe present invention and may also be set to receive a transmitted frameand obtain channel estimation information and channel state informationby interpreting a field value included in the frame. The processor 910may be set to feed back the channel estimation information and thechannel state information through a specific bandwidth on the basis ofinformation included in the NDPA frame or the NDP frame or both. Theprocessor 910 may be set to perform subsequent channel soundingaccording to a received feedback frame. The processor 910 may be set toimplement the embodiments of the present invention described above withreference to FIGS. 4 and 8.

The processor 910 and/or the transceiver 930 may include anapplication-specific integrated circuit (ASIC), a separate chipset, alogic circuit, and/or a data processing unit. When the embodiment of thepresent invention is implemented in software, the aforementioned methodscan be implemented with a module (i.e., process, function, etc.) forperforming the aforementioned functions. The module may be stored in thememory 920 and may be performed by the processor 910. The memory 920 maybe located inside or outside the processor 910, and may be coupled tothe processor 910 by using various well-known means.

What is claimed:
 1. A method for a channel sounding in a wireless localarea network, the method comprising: receiving, by a station, a 20 MHznull data packet announcement (NDPA) frame and at least one duplicate 20MHz NDPA frame, the at least one duplicate 20 MHz NDPA frame being aduplicate of the 20 MHz NDPA frame; receiving, by the station, a nulldata packet (NDP) following the 20 MHz NDPA frame and the at least oneduplicate 20 MHz NDPA frame, wherein a bandwidth over which the 20 MHzNDPA frame and the at least one duplicate 20 MHz NDPA frame aretransmitted is same as a bandwidth over which the NDP is transmitted;and transmitting, by the station, a report frame for the channelsounding, wherein the 20 MHz NDPA frame and the at least one duplicate20 MHz NDPA frame comprise bandwidth information indicating thebandwidth over which the 20 MHz NDPA frame and the at least oneduplicate 20 MHz NDPA frame are transmitted.
 2. The method of claim 1,wherein the 20 MHz NDPA frame and the at least one duplicate 20 MHz NDPAframe comprise a signal field and a data field, and wherein thebandwidth information is included in the signal field.
 3. The method ofclaim 1, wherein the report frame comprises information about abeamforming feedback matrix and information about a channel used tocreate the beamforming feedback matrix.
 4. A station configured toperform channel sounding in a wireless local area network, the stationcomprising: a transceiver configured to transmit and receive signals;and a processor operatively connected to the transceiver and configuredto: receive a 20 MHz null data packet announcement (NDPA) frame and atleast one duplicate 20 MHz NDPA frame, the at least one duplicate 20 MHzNDPA frame being a duplicate of the 20 MHz NDPA frame; receive a nulldata packet (NDP) following the 20 MHz NDPA frame and the at least oneduplicate 20 MHz NDPA frame, wherein a bandwidth over which the 20 MHzNDPA frame and the at least one duplicate 20 MHz NDPA frame aretransmitted is same as a bandwidth over which the NDP is transmitted;and transmit a report frame for the channel sounding, wherein the 20 MHzNDPA frame and the at least one duplicate 20 MHz NDPA frame comprisebandwidth information indicating the bandwidth over which the 20 MHzNDPA frame and the at least one duplicate 20 MHz NDPA frame aretransmitted.
 5. The station of claim 4, wherein the 20 MHz NDPA frameand the at least one duplicate 20 MHz NDPA frame comprise a signal fieldand a data field, and wherein the bandwidth information is included inthe signal field.
 6. The station of claim 4, wherein the report framecomprises information about a beamforming feedback matrix andinformation about a channel used to create the beamforming feedbackmatrix.